In display devices such as liquid crystal display devices, color filters are widely used for the purposes of color image display, reflectance reduction, contrast adjustment, spectral characteristic control, and the like. A color filter is formed by arranging colored pixels in a matrix on a substrate. Methods for forming such colored pixels on a substrate include, for example, printing and photolithography.
FIG. 7 is an enlarged view of pixels of a color filter, and FIG. 8 is a cross-sectional view of the pixels, taken along a line X-X in FIG. 7.
The color filter shown in FIGS. 7 and 8 includes a substrate 50, a lattice-shaped black matrix 21 formed on the substrate 50, colored pixels 22, and a transparent conductive film 23. The black matrix 21 has a light-shielding property, defines the positions of the colored pixels 22 on the substrate 50, and makes the size of the colored pixels 22 uniform. In addition, when the color filter is used in a display device, the black matrix 21 blocks unnecessary light to achieve a high-contrast, even, and uniform image quality. The colored pixels 22 function as a filter for reproducing various colors.
A color filter is formed as follows. Firstly, a black photoresist is applied to the substrate 50, and exposed to light through a photomask and then developed, thereby forming a black matrix 21. Next, a color resist is applied to the substrate 50, and exposed to light through a photomask and then developed, thereby forming colored pixels 22. The process of forming colored pixels 22 is repeated until colored pixels 22 of all colors are formed on the substrate. Further, ITO (Indium Tin Oxide) is deposited by sputtering over the entire surface of the substrate 50 so as to cover the black matrix 21 and the colored pixels 22, thereby forming a transparent conductive film 23.
In mass production of the above-described color filter, it is general to form an array of a plurality of color filters on a single large substrate. For example, four color filters each having a diagonal of 17 inches can be formed on a glass substrate having a size of about 650 mm×850 mm.
As described above, in order to form a plurality of color filters on a single substrate, exposure has been popularly performed by using a photomask of approximately the same size as the substrate, on which a plurality of mask patterns corresponding to all the color filters are formed (for example, in the above-described example, a photomask on which four mask patterns corresponding to color filters each having a diagonal of 17 inches are formed). According to this method, patterns corresponding to all the mask patterns on the photomask are simultaneously formed on the substrate by a single exposure (so-called one-shot exposure).
However, the size of the photomask is increased with an increase in the size of the color filter. Thereby, the manufacturing cost of the photomask increases, and moreover, a problem of deflection of the photomask may occur due to its own weight at the time of exposure.
So, in order to resolve the problems of high cost and deflection due to an increase in the size of the photomask, an exposure method has been adopted, in which a plurality of exposures are performed by using a single photomask capable of simultaneously exposing a plurality of color filters, while changing the position of the photomask opposed to a substrate. For example, when the size of the substrate became about 730 mm×920 mm (the fourth generation), a single-axis step exposure method was adopted, in which exposure is repeated with the substrate being moved in steps along one direction with respect to a photomask. When the size of the glass substrate became about 1000 mm×1200 mm (the fifth generation), an XY (two-axis) step exposure method (step and repeat method) was adopted, in which exposure is repeated with the substrate being moved in steps along two directions with respect to a photomask.
FIG. 9 is a plan view illustrating an example of manufacturing of color filters by the XY step exposure method.
On a substrate 50, first to sixth exposure regions 1Ex to 6Ex are provided, in which six (two rows×three columns) color filters are to be exposed. The substrate 50 is placed on an exposure stage 60, and is freely movable in the X and Y directions.
Firstly, exposure is performed with a photomask PM being overlapped with the first exposure region 1Ex to form a mask pattern of the photomask PM in the first exposure region 1Ex. Thereafter, the substrate 50 is moved by a distance Py in the positive direction of the Y axis to overlap the photomask PM with the second exposure region 2Ex, and a pattern of the photomask PM is formed in the second exposure region 2Ex. Next, the substrate 50 is moved by a distance Px in the positive direction of the X-axis to overlap the photomask PM with the third exposure region 3Ex, and a pattern of the photomask PM is formed in the third exposure region 3Ex. Thereafter, in a similar manner to above, exposure is repeated with the substrate 50 being moved in the X direction or the Y direction, thereby forming patterns in the fourth to sixth exposure regions 4Ex to 6Ex.
The use of the XY 2-axis step exposure method resolves the problem of an increase in manufacturing cost due to an increase in the size of the photomask, and the problem of deflection of the photomask due to its own weight. However, if the size of the substrate is further increased (for example, about 1500 mm×1800 mm (the sixth generation) or about 2100 mm×2400 mm (the eighth generation)), the color filters themselves formed on the substrate are also increased in size, which eventually causes an increase in the size of the photomask. As a result, the problems of high cost and deflection of the photomask occur again.
So, an exposure method is attempted, in which exposure is continuously performed by using a photomask smaller than a single color filter, while transferring a substrate.
FIG. 10 is a plan view illustrating a slit exposure method. FIG. 11 is a cross-sectional view taken along a line X-X in FIG. 10. FIG. 12 is a partially enlarged view of a mask pattern of a photomask shown in FIG. 10. FIG. 13 is a partially enlarged view of stripe patterns exposed by the slit exposure method. In FIG. 11, part (a) shows a state where exposure of a first exposure region is started, and part (b) shows a state where exposure of the first exposure region is completed.
As shown in FIGS. 10 and 11, in the slit exposure method, a photomask PM2, which is smaller in size than a first exposure region 1Ex of a substrate 50 placed on an exposure stage 60, is disposed between the substrate 50 and a light source (not shown). The exposure stage 60 is movable at a constant speed in the horizontal direction of the figure, and further, is movable in steps in the vertical direction of the figure along the Y axis. As shown in FIG. 12, the photomask PM2 has a slit S for exposing a portion of a pattern formed in the first exposure region 1Ex. In the longitudinal direction Ls of the slit S, a plurality of openings 51 are aligned at predetermined intervals Pi. The width and length of each opening 51 are Wi and Li, respectively.
When exposing the first exposure region 1Ex, as shown in FIGS. 10 and 11(a), the photomask PM2 is placed on the left end of the first exposure region 1Ex. Then, while irradiating the photomask PM2 with a light beam from the light source, the substrate 50 is continuously transferred leftward in FIG. 10 along the X axis, until reaching the state shown in FIG. 11(b). As a result, as shown in FIG. 13, stripe patterns each having a width Wi and an interval Pi are formed, on the substrate 50, extending in the substrate transfer direction (the horizontal direction of FIG. 10).
After the exposure of the first exposure region, the exposure stage 60 is moved by a distance Py in the positive direction of the Y axis in FIG. 10 to align the photomask PM2 to an exposure start position in the second exposure region. Then, stripe patterns are formed in the second exposure region by performing continuous exposure similar to that performed on the first exposure region.
Thus, the slit exposure method realizes large-area exposure as well as a size reduction of the photomask.
FIG. 14 is a partially enlarged view of a color filter manufactured by the slit exposure method.
In the color filter shown in FIG. 14, stripe colored patterns extending in the X direction are formed on a glass substrate on which a lattice-shaped black matrix 21 is formed, thereby forming red colored pixels 22R, green colored pixels 22G and blue colored pixels 22B. In the Y-axis direction, a set of red, green, and blue colored pixel lines is repeatedly formed at a pitch Pi.
The patterns formed by the slit exposure method are limited to stripe patterns that are continuous in the substrate transfer direction (X direction in FIG. 14). Therefore, the slit exposure method is not applicable to formation of nonlinear patterns such as rectangle colored pixels or cylindrical spacers. As a method similar to the slit exposure method, a pulsed light exposure method has been proposed, in which nonlinear patterns are formed by intermittently emitting a light beam from a light source (by repeating turn-on and turn-off of the light source) (refer to Patent Literature 1, for example).
The pulsed light exposure method is fundamentally identical to the slit exposure method described with reference to FIGS. 10 and 11. However, in the pulsed light exposure method, instead of continuously emitting a light beam from the light source, a light beam is emitted from the light source at an instant when a pattern formation region on a moving substrate passes beneath an opening of a photomask. By repeating the instantaneous light emission at predetermined intervals, a plurality of mask patterns are intermittently printed. Since the light-emission time per pulse is about several tens of microseconds, an exposure deviation, which is caused by that the substrate moves during irradiation, falls within an allowable range.
FIG. 15 is a partially enlarged view of a color filter manufactured by the pulsed light exposure method.
In the color filter shown in FIG. 15, rectangle colored patterns are formed on a substrate on which a black matrix 21 is formed, thereby forming red colored pixels 22R′, green colored pixels 22G′, and blue colored pixels 22B′. The width and length of each colored pixel are Wi and Li, respectively. The rectangle colored pixels of each color are formed at an equal pitch Pi-2, discontinuously in the X-axis direction. In the Y-axis direction, lines of colored pixels are repeatedly aligned in order of red, green, and blue at an equal pitch Pi. In the X-axis direction, the respective colored pixels are repeatedly arranged at the pitch Pi-2. In the Y-axis direction, a set of adjacent colored pixel lines of red, green, and blue is repeatedly arranged at the pitch Pi.
FIG. 16 is a plan view illustrating an example of a plurality of types of color filters formed on the same substrate.
While in the above description a plurality of color filters of the same type are formed on a single substrate, there are cases where a plurality of color filters of different types are formed on a single substrate. In the example of FIG. 16, four color filters CF-A and three color filters CF-B are formed, and the color filters CF-B are different from the color filters CF-A in the size of colored pixels or in the finished size of color filters. By adopting a method of forming a plurality of color filters of different types together on a single substrate, a blank space (a region indicated by “B”), which is generated when a plurality of color filters CF-A are formed in a region indicated by “A” in FIG. 16, can be filled with color filters CF-B smaller than the color filters CF-A. Such an effective use of the blank space enables a reduction in manufacturing cost per color filter. Moreover, supply of a plurality of color filters of different types can be started in a short time.